This invention relates to the field of electrophotography, i.e., xerography, and particularly to a means for reducing carryout of the carrier constituent of a developer mix by a photoconductor drum which includes a drum seal.
In the electrophotographic or electrostatic printing process a photoconductor, bearing a latent electrostatic image, is developed by applying a developer mix including electroscopic toner which is electrostatically carried by carrier beads. The developer includes means to agitate the toner and carrier so as to triboelectrically charge the carrier to a first polarity and the toner to the opposite polarity.
One form of photoconductor is a flexible sheet carried on the rigid surface of a drum. The photoconductor is stored in flexible strip form on supply and take-up rolls located within the drum's interior. The portion of the photoconductor extending between the two rolls encircles the drum and is active in the xerographic process. To change this active portion, a length of the photoconductor is advanced from the supply roll to the take-up roll. The drum's surface is formed with an axially extending opening or slit whereat the photoconductor enters and exits the drum's interior. This slit is closed by a seal strip to prevent developer mix, i.e., toner and carrier, from entering the interior of the drum. U.S. Pat. No. 3,588,242, issued to R. A. Berlier et al is an example of such a photoconductor.
While this drum seal is generally satisfactory, it has been found that developer mix carryout, and particularly carrier carryout by the drum seal as the seal leaves the developer, is improved by the present invention. Specifically, the present invention contemplates changing the bias voltage on the developer's development electrode to create an electrical field which operates to reduce the force with which the carrier is presented the drum seal while the drum seal is within the developer.
The prior art teaches diverse reasons for changing a developer's bias voltage. In one instance the electric field between a paper supported photoconductor and a magnetic brush developer is adjusted as a function of the conductivity of the photoconductor to repell unwanted toner from the photoconductor's background area.
Other prior art teaches that charged toner can be transferred from one member to another. For example, the reversal of a cleaning member's bias voltage operates to effect toner removal from the cleaning member to a drum-carried serrated plate during passage of the plate past the cleaning member.
Yet other prior art teaches the concept of selectively changing the bias applied to a developer to selectively develop or clean the photoconductor.
The preferred embodiment of the present invention relates to a magnetic brush developer and to the concept of altering, i.e., reducing, the brush roll's bias voltage in synchronism with passage of the photoconductor drum seal adjacent a developing station, to thereby reduce the number of magnetic beads which are carried out of the developing station by the drum seal.
While the present invention will be described in the environment of its preferred embodiment, namely a magnetic brush developer, its use is not restricted thereto, but rather the present invention is believed to have utility with any developer having a development electrode or its equivalent.
More specifically, and only by way of a specific example, the magnetic brush developer may be used to supply negatively charged carrier beads and positively charged toner particles to the incrementing drum photoconductor's negatively charge latent image. Toner is thus electrostatically applied to the latent image to form a visible image. The physical discontinuity represented on the drum's surface by the drum seal operates to physically pick up negative carrier beads. The magnetic brush roll is biased negative when the photoconductor is passing the brush roll, to function as a development electrode, and is biased less negative, i.e., substantially to zero potential, or perhaps positive, when the drum seal is passing the brush roll, to thereby reduce the force with which the negative carrier beads are presented to the drum seal.
The foregoing and other features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention, as illustrated in the accompanying drawing.